FIG. 1 schematically shows a general conventional loading/unloading hydraulic circuit in a fork lift truck. In such a hydraulic circuit, a fixed displacement type hydraulic pump 1 sucks up hydraulic fluid from an oil tank 3 to supply or discharge the hydraulic fluid to or from a lift cylinder 5 through a control valve 7.
In the above-mentioned hydraulic circuit, since the pump is usually a fixed displacement type, the lifting speed of a piston rod 9 of the lift cylinder 5, i.e., the lifting speed of a fork (not shown) attached to the piston rod 9, is generally constant whether the fork lift truck is under a load or not. This lifting speed is st so that it will be optimal when cargos are loaded on the fork. As a result, for example, while the fork lift truck is going to pick up a cargo from a high location, the lifting speed of the fork is comparatively low in spite of being unloaded, thereby generating needless waiting time and decreasing operating efficiency.